The invention relates to a semiconductor device comprising a semiconductor body which is provided at a surface with a MOS transistor having a gate which is electrically insulated from an underlying channel region by an intermediate gate dielectric, and, in a neighboring region of a first conductivity type, a surface zone of a second, opposite conductivity type is formed which is conductively connected to the gate and which forms a protection diode with the adjacent region of the semiconductor body, via which protection diode electric charge is removed from the gate before the gate dielectric is damaged as a result of electrostatic discharge.
Such a device is known, inter alia, from the patent U.S. Pat. No. 5,366,908.
In the manufacture of CMOS and BICMOS type integrated circuits, it is customary to grow, in a first phase of the process, the thin gate oxide having a specific thickness of, for example, 10 nm, whereafter the gates, customarily of doped polycrystalline silicon, are provided thereon, and subsequently the source and drain zones of the transistors are formed in the semiconductor body. In a next phase, the whole structure is provided with a dielectric layer, for example an oxide layer, whereafter the wiring in the form of one or more metal layers is formed in order to connect the different components of the circuit to each other and to bonding pads enabling external connections to be made. As the dimensions are becoming smaller and smaller (sub-micron) and the number of components as well as the complexity of the circuits are increasing continuously, the number of metal layers tends to increase and amounts to at least four in current circuits.
During the wiring process, connections can be made which are electrically floating until they are connected again by the last wiring layer. In the meantime, however, electric charge caused, for example, by plasma etching, may accumulate on these floating connections. If one of these connections is coupled to the polygate of a transistor, the charge may leak to the substrate via the thin gate oxide, whereby it may exceed a maximum value and, thus, cause irreparable damage to the gate oxide.
To preclude premature breakdown of the gate oxide, said patent U.S. Pat. No. 5,366,908 already proposes to connect the polygate to a diode formed in the semiconductor body. If, during the production process, electric charge is stored on the electrically floating gate, this charge can be removed via the diode before causing damage to the gate oxide.
Since the number of transistors may be very large, it is important that the dimensions of the diode are as small as possible so as to preclude that the surface of the circuit becomes too large. In addition, it is important that the diode should be compatible with the rest of the circuit and, preferably, can be manufactured with the same process steps as the transistors. In the known device, the connection between the diode and the polygate takes the form of a CS contact in which the poly is directly deposited, at the location of the diode, on the locally exposed surface of the semiconductor body. In the manufacture of this device, the gate oxide grown is provided with a thin polylayer which serves to protect said gate oxide. A window which defines the position of the diode is subsequently provided in this polylayer and the underlying gate oxide. Next, a second polysilicon layer is deposited which contacts the silicon body at the location of the window. By means of a doping step, the polylayer is provided with an n-type doping, and, simultaneously, the protection diode is formed at the location of the CS contact in the semiconductor body. In a subsequent series of process steps, the poly is provided with a pattern, whereafter the source and drain zones of the transistors are formed in a customary manner. The process includes process steps, such as the provision of the first, protective polylayer and the provision of the window at the location of the diode, which process steps are required only in connection with the diode and add to the complexity of the production process.